Field
The present invention relates to relaying frames by a redundant port and more particularly to duplicate detection in communications involving redundancy.
Description of the Related Art
The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.
Several communications protocols provide redundant communications using duplicate messages thereby ensuring that the message is received by the destination. IEC 62439-3 Edition 2 defines two redundancy protocols: Parallel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR) protocol.
Both PRP and HSR operate on Ethernet link layer, which is layer 2 of the Open Systems Interconnection (OSI) model. The operation of both PRP and HSR is fully transparent for the layers above: while the source sends duplicates, the receiver must be able to remove duplicates and it delivers only a single frame to the layers above.
The PRP and HSR protocols define that a source address together with a sequence number shall be used for detecting duplicates. The both HSR and PRP include both a source Medium Access Control (MAC) address and a 16 bit sequence number in every frame that is sent. The HSR/PRP source node adds the sequence numbers to the frames. A sequence number is source MAC address specific, i.e. every source has its own running sequence number that is incremented by one for every frame sent to the PRP/HSR network. The HSR/PRP node sends the same data duplicated via two paths with the same sequence number.
When the two paths do not have faults, the duplicate frames are received via both paths at the destination node. However, since only one frame is needed at the destination node, the duplicates need to be discarded. Failing to identify duplicates and to discard correct frames may prevent optimal operation of upper layer protocols, e.g. those on top of UDP or layer 2 and those that have not been designed to deal with duplicates. Furthermore, duplicate frames increase the network load and the load of the nodes processing the received frames.